We report the most sensitive direct upper limit set on the mass my of the electron antineutrino. Our measurements of the shape of the fi decay spectrum of free molecular tritium yield, under the assumption of no new physics other than that of mass, a central value for mv of -147 ±68 ±41 eV^, which corresponds to an upper limit of 9.3 eV (95% confidence level) on mv. The result is in clear disagreement with a reported value of 26(5) eV.PACS numbers: 23.40.Bw, 14.60.GhThat the mass of the electron neutrino (or antineutrino) could be determined from the shape of p spectra has been known since Fermi's formulation of the theory of p decay. A group at the Institute for Theoretical and Experimental Physics (ITEP) in Moscow have reported [1] from their studies of the tritium p spectrum that the Ve mass lies in the range 17-40 eV, with revolutionary implications for particle physics and cosmology. Not only are massive neutrinos incompatible with the otherwise successful minimal standard model of particle physics, a neutrino mass in that range would be sufficient both to close the Universe gravitationally and to account qualitatively for observational evidence for dark matter. Other experiments [2][3][4], however, do not support the ITEP claim.When tritium decays to ^He, the orbital electrons redistribute themselves over the set of eigenstates of the residual molecule. The resulting energy spread impressed on the outgoing p must be very precisely calculated if serious errors in interpreting the data are to be avoided. Such calculations can be carried out with some confidence for atomic and molecular tritium, in contrast to the multielectron solid sources used heretofore [1][2][3]. Our experiment at the Los Alamos National Laboratory is the first to make use of a gaseous source of T2 to capitalize on the simplicity of the two-electron system. The gaseous source also confers minimal, well understood energy-loss corrections, and freedom from backscatter; together with detailed measurements of the instrumental resolution function, energy-loss, energy efficiency, and other effects, it has made possible a reliable mass measurement at the 10-eV level.In an earlier paper [4] we described our apparatus briefly and reported our initial result, rriy < 27 eV. The source is a tube fed at its midpoint with a steady flow of T2 gas and placed on the axis of a solenoidal magnetic field whose strength decreases monotonically toward the extraction end. Electrons from p decay near the axis are guided to the object point of a toroidal-field magnetic spectrometer of 5-m focal length. The most significant of many improvements [5] made recently are the elimina-tion of electron trapping in the source and replacement of the single-element proportional counter in the spectrometer by an octagonal array of Si microstrip detectors, each with twelve pads. Signals from pads at the same location along the dispersion axis are combined to form twelve simultaneously acquired spectra, each independently calibrated by a ^^Kr'" spectrum similarly formed.Tritium...

Collagen gels are used extensively for studying cell-matrix mechanical interactions and for making tissue equivalents, where these interactions lead to bulk deformation of the sparse network of long, highly entangled collagen fibrils and syneresis of the interstitial aqueous solution. We have used the confined compression test in conjunction with a biphasic theory to characterize collagen gel mechanics. A finite element method model based on our biphasic theory was used to analyze the experimental results. The results are qualitatively consistent with a viscoelastic collagen network, an inviscid interstitial solution, and significant frictional drag. Using DASOPT, a differential-algebraic equation solver coupled with an optimizing algorithm, the aggregate modulus for the collagen gel was estimated as 6.32 Pa, its viscosity as 6.6 ϫ 10 4 Pa s, and its interphase drag coefficient as 6.4 ϫ 10 9 Pa s m Ϫ2 in long-time ͑5 h͒ creep. Analysis of short-time ͑2 min͒ constant strain rate tests gave a much higher modulus ͑318.3 Pa͒, indicating processes that generate high resistance at short time but relax too quickly to be significant on a longer time scale. This indication of a relaxation spectrum in compression is consistent with that characterized in shear based on creep and dynamic testing. While Maxwell fluid behavior of the collagen network is exhibited in shear as in compression, the modulus measured in shear was larger. This is hypothesized to be due to microstructural properties of the network. Furthermore, parameter estimates based on the constant strain rate data were used to predict accurately the stress response to sinusoidal strain up to 15% strain, defining the linear viscoelastic limit in compression.

Los Alamos Nahonal Laboratory ib operated by the University of Caldornia for the United States Department of Energy under contract W-7405-ENG-36.

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Electron energy spectra have been measured that result from A'-shell ionization of Kr by two different mechanisms: (1) photoionization and (2) internal conversion in the decay of the isomeric state of 83 Kr. It is demonstrated experimentally that these spectra, including satellites on the low-energy side of the primary ls-electron peak, are identical. A theoretical interpretation of the identity of the spectra is given. The spectra agree well with a relativistic many-electron calculation in which the satellites are attributed to excitation and ionization of M and TV electrons during the ^-ionization process.